Input 如何访问不同SUB中的不同变量？

我一直在尝试在VisualStudio2012 Ultimate上制作计算器。我一直在尝试创建不同的按钮，一个询问第一个号码，第二个询问第二个号码，第三个询问操作员，最后一个在消息框中显示答案。但每当我尝试从另一个子系统调用整数时，它就是不起作用。我已经试着公开了。这是我的密码：

Public Class Form1

    Public Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim prompt1, title1 As String
        Dim a As Integer
        prompt1 = "Enter First Number : "
        title1 = "First Number"
        a = InputBox(prompt1, title1)
    End Sub

    Public Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        Dim prompt2, title2 As String
        Dim b As Integer
        prompt2 = "Enter Second Number : "
        title2 = "Second Number"
        b = InputBox(prompt2, title2)
    End Sub

    Public Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
        Dim prompt3, title3 As String
        Dim op As String
        Dim ans As Integer
        prompt3 = "Enter Operator : "
        title3 = "Operator"
        op = InputBox(prompt3, title3)
        If op = "+" Then
            ans = a + 
        End If


    End Sub

    Public Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click


    End Sub
End Class

正如你所看到的，我还没有完成它，因为我不能在第三个按钮中调用a

有什么想法吗

---

如果我搞砸了，请指出。

在button1之外定义变量a

Public Class Form1
    Dim a As Integer
Public Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
 Dim prompt1, title1 As String
 prompt1 = "Enter First Number : "
 title1 = "First Number"
 a = InputBox(prompt1, title1)
End Sub

现在可以使用变量“a”